In the all-pixel sequential transfer operation, the optical signal can be read after reading the reset signal, and so it is possible to perform the analog correlated double sampling (CDS) used to read the optical signal by setting the reset signal as a reference. However, rolling distortion will occur in the all-pixel sequential transfer operation.
Thus, the all-pixel simultaneous transfer operation in which distortion does not occur in principle has been developed.
The all-pixel simultaneous transfer operation is roughly divided into two modes. In the first mode that is one of two, a memory (MEM) is arranged inside a pixel circuit, and the optical signal is transferred from a photodiode (PD) to the MEM simultaneously for all pixels. Then, the optical signal is read from the MEM using the all-pixel sequential transfer operation described above.
However, the implementation of the first mode is necessary to increase the number of pixel circuits, and so the increase in the number of pixel circuits will cause reduction in the area for receiving the light incident on the PD.
Thus, as the second mode, the FD accumulation-type all-pixel simultaneous transfer operation is developed in which electric charges are simultaneously accumulated in a floating diffusion (FD) instead of MEM, and an optical signal is read from the FD by the all-pixel sequential transfer operation.
Furthermore, the FD accumulation-type all-pixel simultaneous transfer operation has two types of modes.
In the first mode, after resetting the FD, the reset signal is not read, the optical signals are transferred collectively, the optical signal is read, the same pixel is reset again by setting the read optical signal as a reference, and the difference from the reset signal is read.
In this first mode, the analog CDS can be performed. However, the kTC noise (noise that changes every reset and its value is proportional to the square root of kTC, where k is Boltzmann's constant, T is temperature, and C is capacitance) of the pixel fails to be removed in principle, and so different noise signals will be superimposed on both the optical signal and the reset signal.
In addition, in the second mode, the reset signals of all the pixels are read, are stored in the frame memory, and are exposed. Electric charges are accumulated by the PD using photoelectric conversion and then are transferred simultaneously from the PD to the FD. The optical signal is read from the FD by the all-pixel sequential transfer operation.
In this second mode, although the frame memory is necessary, the analog CDS can be performed, and the same kTC noise is included in each of the reset signal and the optical signal, so it is possible to cancel out the kTC noise by the analog CDS, as compared with the first mode.
However, in the all-pixel sequential transfer operation, the reset signal serving as a reference is read after reading the optical signal. Thus, in the configuration described above, an operating point serving as a reference in the column amplifier is not determined and an appropriate pixel signal is likely to fail to be output.
Thus, a technology is developed in which a circuit that generates the reference voltage is provided (refer to Patent Literature 1). In this technology, the image capturing is performed in the same sensor using the all-pixel sequential transfer operation during image capturing and using the FD accumulation-type all-pixel simultaneous transfer operation during the auto exposure (AE) or auto focus (AF), which is based on the reference voltage that is output from the circuit.
In other words, in the FD accumulation-type all-pixel simultaneous transfer operation, a signal serving as a reference at the time of resetting is read after reading the optical signal. Thus, in the description of Patent Literature 1, the column amplifier operation can be achieved by outputting a fixed reference voltage using the circuit that performs the auto-zero (AZ) operation.